This invention relates to polyesters and/or carbonates that have modified rheological properties and methods for making those polyester and/or polycarbonates.
Certain monocyclic compounds can be polymerized to form polyesters or polycarbonates. Examples of those monocyclic esters include dioxanones (such as p-dioxanone), lactones (such as ε-caprolactone or 4-valerolactone), dioxan(dione)s (such as glycolide, lactide or tetramethyl-1,4-dioxan-2,5-dione), carbonates such as ethylene carbonate and trimethylene carbonate, and ester-amides (such as morpholine-2,5-dione). Commercial interest in these polymers, particularly polylactide polymers (also known as polylactic acid, or PLA), is rapidly increasing. Unless modified in some way, these polyesters are linear molecules and therefore thermoplastic materials. They are useful for making a variety of films, fibers and other products. In the case of PLA, these polymers offer the significant advantages of being derived from renewable resources (lactic acid can be prepared from plant carbohydrates such as dextrose) and of being biodegradable. However, the rheological properties of these polymers are such that they can be difficult to process in certain applications. This difficulty in processing has so far limited the applications for which these polymers can be used. For example, in extrusion coating, poor rheological properties lead to phenomena such as neck-in and draw instability (draw resonance and edge weave). Poor rheological properties make it very difficult to make blow molded articles at all, and cause extruded foams to collapse because operating windows are extremely narrow.
The rheological property of primary interest is often melt elasticity, which is often expressed as “melt strength”. Broadly speaking, it is desirable that a thermoplastic polymer forms a melt having a reasonably low shear viscosity so that it can be processed readily, but at the same time the molten polymer must possess enough strength that, once formed into a desired shape, it can hold that shape and in some instances even be worked with until it has time to cool and harden. As a general rule, melt strength can be increased in a thermoplastic resin by increasing the molecular weight. However, this also increases the shear viscosity so that the benefits of improved melt strength are offset by the increased force that is needed to shape the polymer in the first place. The increased force needed requires, at minimum, higher power consumption to process the polymer. In some cases this means that heavier, more expensive equipment is needed, or else processing rates must be reduced. In addition, increasing molecular weight tends to increase the processing temperatures that are required, and this exacerbates polymer degradation.
Accordingly, attempts to improve the processing characteristics of these polymers have tended to focus on introducing branching through some mechanism. In the case of PLA, for example, it has been attempted to copolymerize lactide with an epoxidized fat or oil, as described in U.S. Pat. No. 5,359,026, to treat PLA with peroxide, as described in U.S. Pat. Nos. 5,594,095 and 5,798,435, and to use certain polyfunctional initiators as described in U.S. Pat. Nos. 5,210,108 and 5,225,521 to Spinu, GB 2277324 and EP 632 081.
Unfortunately, none of these methods is entirely satisfactory. In some cases, the rheological properties of the polymer are not improved as much as desired. Good rheological improvements can be obtained in other cases but the manufacturing process is difficult to control, which makes it difficult to make the desired product in a reproducible way. Sometimes, the branching agent does not copolymerize well with the monocyclic ester or carbonate. This is particularly true in the case of lactide. In still other cases, the steps required to induce branching can interfere with the polymerization. This can lead to increased polymerization times, uneven product quality, and other problems.
It would be desirable to provide a polymer of a monocyclic ester (or corresponding hydroxy acid) and/or monocyclic carbonate, which polymer has improved rheological properties, yet remains processable at temperatures below that at which the polymer begins to degrade significantly. Biodegradability would be a further advantage. It is further desirable to provide a convenient process by which monocyclic esters and/or carbonates can be polymerized to form polymers having improved rheological properties, and in particular a process which is easily controllable to form polymers having predictable and reproducible rheological properties.